Genes involved in the determination and differentiation of adult tissues in Drosophila melanogaster have been identified by genetic interactions with homeotic and segmentation genes already known to be required in these tissues. Many new genes required for transcriptional activation of developmental genes have been identified. One of these new genes, brahma, encodes a large nuclear protein conserved from yeast to man. brahma mutations have in turn been used to identify other interacting genes required for transcriptional activation. The osa gene, which shows allele-specific interactions with brahma mutations, has been cloned by transposon tagging, and encodes a single 3 kb transcript. We have shown that maternal expression of both brahma and osa is essential for early embryogenesis, while zygotic expression of both is essential late in embryogenesis and during larval growth. Both genes are expressed primarily in the nervous system at later embryonic stages. This is also the tissue that expresses the highest levels of the homeotic genes that brahma and osa regulate. brahma and osa function are also required in the larval tissues for expression of the hedgehog gene, which encodes a secreted protein involved in cell-cell signaling during development. The RNA polymerase II transcription factor TFIID is a complex of at least nine polypeptides. We have isolated mutations in the Taf60 gene that encodes the 60 kDa subunit of this protein complex. The Taf60 mutations have allowed us to determine several interesting features about the function of this protein complex in development. Maternal expression of the gene is required for oocyte formation. This maternal expression appears to be sufficient for normal embryogenesis. Although zygotic expression is essential for viability, it is not required until the larval stages. Our dosage studies with the Taf60 mutations also suggest that while the TFIID complex may be required for transcriptional activation, its availability is probably not a limiting factor for transcriptional regulation.